This invention relates to vehicle suspensions and, more particularly, to strut type vehicle suspensions in which one or more spring members are employed in combination with shock absorbers or frictional damping means. While three presently preferred embodiments of the invention are illustrated and described herein for application to rubber tired vehicles, the invention is not limited to such applications and may be used with other types of vehicles, other suspensions, and even in non-vehicular applications and environments.
Strut type suspension systems are especially adaptable for use with modern lightweight vehicles, but in general provide unsatisfactory ride qualities over the entire range of vehicle load conditions. Typical suspension struts, such as the MacPherson strut, are made up of conventional helical coil springs in combination with hydraulic shock absorbers. The ride frequency obtained with these, however, tends to be highly load sensitive since it is a function of the spring rate of the coil springs and sprung mass of the vehicle. Although it is possible to obtain a desirable ride frequency at a single predetermined load, the coil spring provides a spring force which is directly proportional to load, based upon its constant spring rate, causing the ride frequency to vary when the vehicle is loaded above or below the predetermined load. Consequently, the ride frequency often degrades when the vehicle is loaded at loads other than the predetermined load.
In modern lightweight vehicles, the load conditions vary substantially, often to the extent that the vehicle has a fully loaded spring mass which is 20 percent to 30 percent greater than its empty sprung mass. In these vehicles, the predetermined load typically is selected to approximate an optimum load which occurs when the vehicle is loaded at a median load within this range. Thus, the ride frequency obtained is satisfactory only when the vehicle is loaded at or near optimum load, but tends to become higher than desirable when the vehicle is light, and lower than desirable when it is loaded. This often leads to passenger discomfort, bottoming out of the springs, and even occasional breakage of the springs. In most practical cases, therefore, conventional suspension struts such as those discussed above provide uncomfortable ride either at light load conditions, heavy load conditions, or both, depending upon the optimum load selected.
Suspension struts of the type described tend to provide insufficient damping at all practical load conditions experienced in light vehicles for similar reasons. Since the damping coefficient is inversely proportional to the sprung mass of the vehicle, the damping force applied tends to decline in magnitude with increasing vehicle load. Consequently, the struts often appear over-damped at light loads and under-damped at heavy loads, and therefore are susceptible to further degradation in ride frequency when the vehicle is loaded at other than the optimum load mentioned above. Further, the damping force applied by these struts is velocity dependent so that they vary in stiffness depending upon whether they are subjected to high or low frequency shock loads. This of course tends to compound the undesirable effects of ride frequency degradation from the standpoint of passenger comfort.
Such suspension struts also tend to impair cornering performance when employed as front suspensions due to the reduced stiffness which results from under-damping as described above. Most front suspensions therefore include torsional stabilizers or roll bars which compensate for the loss in strut stiffness due to under-damped conditions when the vehicle is negotiating a curve. Such roll bars and their attendant mounting structure, however, tend to increase the weight, complexity and cost of such suspensions.
Hydraulic shock absorbers used in such suspension struts tend to be unsatisfactory for additional reasons, among which are temperature sensitivity and hence unreliable damping capability due to thinning of the hydraulic fluid with increased temperature, fluid frothing due to air entrapment and fluid leakage. As a consequence, such strut type suspensions require many close-tolerance machined parts, seals, and valves, and an otherwise relatively sophisticated and hence costly construction in order to maintain acceptable performance and life cycle characteristics. This results in increased weight and greater numbers of parts and hence reduced reliability and high cost. Further, the piston and cylinder components of such hydraulic shock absorbers tend to be susceptible to sticking or damage when subjected to bending loads such as those encountered in many automotive applications.
The term "ride frequency" as used herein refers to the frequency at which the sprung mass of a vehicle or other object supported by the suspension strut oscillates in response to application of a force thereto. Although the term may be used hereinafter with respect to oscillatory movement of a rubber tired vehicle, it should be understood that such usage is for illustrative purposes only, and that the invention may be used in other applications and environments, as mentioned above.